When producing a polymer composition the ingredients thereof, such as different polymers, fillers, additives, etc, should be mixed intimately in order to obtain as homogenous a composition as possible. This intimate mixing is done by compounding the ingredients in a compounding machine, continuous or batchwise; the former type can be exemplified by an extruder which may be of the single screw of twin screw type. When the composition comprises two or more different polymers these should be so thoroughly mixed with each other that, ideally they form a completely homogenous polymer blend. To achieve this the polymers are mixed with or without external heating so that they are melted and converted into liquids and the liquid polymers are mixed at sufficiently high shear rates.
Although the conventional way of compounding polymer compositions in many instances leads to acceptable results, problems are encountered when compounding multi-modal polymer compositions, and more particularly multi-modal polymer compositions comprising a low molecular weight ethylene polymer and a high molecular weight ethylene polymer. Thus, when compounding polymer compositions, e.g. for pipes, so called "white spots" occur in the compounded material. These white spots have a size of about 10-50 .mu.m and consist of high molecular weight polymer particles that have not been adequately compounded in the composition. In addition to being disfiguring the white spots may adversely influence the strength of the composition. Further, when compounding polymer compositions, e.g. for the production of film, gel particles with a size of about 0.01-1 mm often occur. These gel particles appear as disfiguring inhomogeneities in the finished film and consist of high molecular weight polymer particles that have not been adequately compounded in the composition.
The above described white spots and gel particles are a serious problem in the polymer industry and a solution to the problem would mean the removal of an obstacle to use otherwise superior multi-modal polymer compositions.
One has tried to solve the problem with gel particles and white spots by applying large amounts of deformation energy to the polymer composition. Normally, this is achieved by compounding the polymer composition at high shear rates and at high temperatures. However, such excessive energy input frequently results in degradation of the polymer especially when the polymer composition comprises polymer materials with large differences in viscosity. For polymer compositions comprising polymers with large differences in viscosity it has been demonstrated that mixing by applying elongational deformation is more effective and favourable than mixing by pure shear deformation.
Mixing by applying elongational deformation as in the present invention is also referred to as "dispersive mixing" as opposed to conventional "distributive mixing". While distributive mixing aims at achieving a homogenous concentration, e.g. by splitting up the stream as many times as possible, dispersive mixing aims at breaking down one phase by applying high shear or tension forces in order to obtain blending of the components of the stream. The blend thus obtained is free from visual inhomogenities (gels), but does not have to satisfy the requirement of homogenous concentration. Dispersive mixing is primarily used when the ratio between the viscosity of the two phases which are to be mixed is higher than 10:1.
As an example of a device for mixing of viscous liquids, such as polymer compositions by elongational deformation may be mentioned U.S. Pat. No. 5,451,106 to Xuan et al., the contents of which is hereby incorporated by reference. This patent discloses a mixer which has a vertical, cylindrical casing which encloses a lower die member and an adjustable, upper die member. Opposed, annular and concentric die member protrusions define alternating concentric slits and chambers. An inlet opening in the side wall of the casing allows liquid polymer to be fed into the device, through the slits and chambers between the dies where the polymer is subjected to elongational deformation, and finally the polymer exits through a central bottom outlet.
The device according to U.S. Pat. No. 5,451,106 has a number of disadvantages. Thus, arranging the inlet on the side of the casing and the outlet in the bottom of the casing results in an uneven material distribution in the device which gives problem with wear due to pressure differences in the mixing chamber. The inlet and outlet arrangement also makes it difficult to connect the mixing device to existing compounding equipment in a simple manner. Further, the geometry of the circular dies with concentric protrusions confines the number of concentric protrusions and thus the elongational deformation if the outer diameter of the device is to be kept within acceptable limits.